see if someone can help me figure this out. i have an old computer with a abit motherboard ix38 quadgt that when i try using the gtx 660 or gtx 760 i get no display at all, but it boots into windows fine because i can hear it get to desk top. now if i use these cards gtx 750ti, 8800gt and the gtx 560 448 cores that i had before it died they all worked. whats the difference between the gtx 660 and gtx 760 and the gtx 750 ti that works on that same computer?

It is very likely that your old Abit motherboard is not compatible with the 600 and 700 series cards. The gtx 750 and 750ti is first gen maxwell and designed to work better on old system and prebuilt systems. Which was one of their strong points to bring back life to older systems. If the gtx 660 and 760 have been tested on a working system and confirmed that there are no issues then I can only suggest looking for the last bios update in hopes to bring more support with newer hardware.